Musnad Abu Ya'la al-Mawsili 3:62

Ubayd Allah narrated to us, Yazid bin Zuray' narrated to us, Shu'bah narrated to us, from Abu Ishaq, from Asim bin Damrah, from Ali, who said: "The Messenger of Allah (ﷺ) performed the Witr prayer during every part of the night; from its beginning, its middle, and its end. Then his Witr ended at the end of the night."